Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6, further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.
Disk drives may employ a head having a suitable write element, such as an inductive coil, and a suitable read element, such as a magnetoresistive (MR) element. The read element may exhibit a non-linear response which may result in a non-linear PES measurement. That is, the measured PES relative to the actual head displacement from the centerline of the target servo track may exhibit a non-linear relationship. The prior art has compensated for this non-linearity by adjusting the measured PES based on a function that linearizes the PES. For example, U.S. Pat. No. 5,946,158 entitled “SELF-PES LINEARITY CALIBRATION METHOD FOR MR HEAD” teaches to adjust the PES based on:z(y)=y+c(y)where y represents the measured PES and c(y) represents a correction value that is computed based on a linearizing function.